Optical sensor apparatus to detect light based on the refractive index of a sample

ABSTRACT

An optical sensor apparatus includes an optically transmissive structure having planar first, second, and third faces, two or more light sources located outside the structure adjacent the first face, and a photodetector array located outside the prism adjacent the first face. The structure, light sources, and photodetector array are configured such that light from the light sources that is totally internally reflected at an optical interface between the prism and a sample outside the structure proximate the second face is reflected at the third face and incident on a portion of the photodetector array that depends on a refractive index of the sample. The light sources are positioned with respect to the structure and photodetector array such that the totally internally reflected light from each light source corresponds to a different range of refractive index of the sample and maps to a corresponding portion of the photodetector array.

FIELD OF THE INVENTION

Embodiments of the present invention are related to optical sensors andmore specifically to optical sensors that measure index of refraction ofa sample by sensing total internal reflection at an interface between anoptical material and the sample.

BACKGROUND OF THE INVENTION

Systems for refractive index measurement of a sample using the criticalangle are well known in the art, as are the principles of physicsunderlying the measurement of critical angle to determine refractiveindex of a medium. When light traveling from a high index medium isincident upon an interface between the high index medium and anothermedium having a lower refractive index at angles of incidence largerthan a critical angle of incidence, total internal reflection may beobserved. The critical angle is a function of the refractive index ofboth media. However, if the refractive index of one medium is known, therefractive index of the other may be determined from a measurement ofthe critical angle θ_(c) using the well-known formula:

${\sin\;\theta_{c}} = {\frac{n_{2}}{n_{1}}.}$

Where n₁ is the refractive index of the high index medium and n₂ is therefractive index for the low index medium. By convention, the criticalangle of incidence is measured with respect to a line perpendicular tothe interface between the two media.

U.S. Pat. No. 6,097,479 describes a sensor for making critical anglemeasurements in which a light source and photodetector array areencapsulated in a light transmissive housing that acts as the high indexmedium. The housing forms a prism having one face in contact with asample, which acts as the low index medium. Light from the light sourceis incident on an interface between the sample and the prism over arange of incident angles. A portion of the light that is incident on theinterface at angles greater than a critical angle undergoes totalinternal reflection and is detected by the photodetector array.Different parts of the photodetector array are therefore illuminated bythe totally internally reflected light depending on the critical angle,which depends on the index of refraction of the prism and of the sample.The pattern of illumination of the photodetector array can be analyzedto determine the index of refraction of the sample.

It is within this context that embodiments of the present inventionarise.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1A is a three-dimensional schematic diagram of an optical sensorapparatus according to an embodiment of the present invention.

FIG. 1B is a side-view schematic diagram of an optical sensor apparatusaccording to an embodiment of the present invention.

FIG. 2 is a three-dimensional graph illustrating a photodetector arraysignal in an optical apparatus according to an embodiment of the presentinvention.

FIG. 3A is a side view schematic diagram illustrating an optical sensorapparatus according to an alternative embodiment of the presentinvention.

FIG. 3B is a side view schematic diagram illustrating an optical sensorapparatus according to another alternative embodiment of the presentinvention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Although the following detailed description contains many specificdetails for the purposes of illustration, anyone of ordinary skill inthe art will appreciate that many variations and alterations to thefollowing details are within the scope of the invention. Accordingly,the exemplary embodiments of the invention described below are set forthwithout any loss of generality to, and without imposing limitationsupon, the claimed invention.

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” etc., may sometimes beused with reference to the orientation of the figure(s) being described.Because components of embodiments of the present invention can bepositioned in a number of different orientations, the directionalterminology is used for purposes of illustration and is in no waylimiting. It is to be understood that other embodiments may be utilizedand structural or logical changes may be made without departing from thescope of the present invention. The following detailed description,therefore, is not to be taken in a limiting sense, and the scope of thepresent invention is defined by the appended claims.

GLOSSARY

As used herein, the following terms have the following meanings.

Coefficient of Thermal Expansion refers to a property of a material thatquantifies a change in one or more physical dimensions of the materialwith change in temperature.

CTE-matched refers to materials having similar coefficients of thermalexpansion (CTE). For the purposes of the present application twomaterials can be said to be CTE-matched if their coefficients of thermalexpansion are within about a factor of 2 of each other.

Dispersion (or optical dispersion) refers to a phenomenon by which awave separates into spectral components with different frequencies, dueto a dependence of the wave's speed on its frequency when the wavetravels in a material. In optics this may be expressed as a dependenceof the refractive index of the material on the vacuum wavelength oflight.

Index of Refraction (or refractive index) refers to an optical propertyof a material that is generally defined as the ratio of the speed oflight in vacuum (or other reference medium) to the speed of light in thematerial.

Infrared Radiation refers to electromagnetic radiation characterized bya vacuum wavelength between about 700 nanometers (nm) and about 100,000nm.

Light generally refers to electromagnetic radiation in a range offrequencies running from infrared through the ultraviolet, roughlycorresponding to a range of vacuum wavelengths from about 1 nanometer(10⁻⁹ meters) to about 100 microns.

Sapphire generally refers an anisotropic, rhombohedral crystal form ofAluminum Oxide (Al₂O₃).

Total Internal Reflection refers to a phenomenon in whichelectromagnetic radiation in a given medium which is incident on aninterface with medium having a lower index of refraction at an anglegreater than a critical angle is completely reflected from the boundary.By convention, the critical angle of incidence is measured with respectto a line perpendicular to the interface between the two media. If theangle of incidence is measured with respect to a line tangent to theinterface, then total internal reflection occurs for angles of incidenceless than the critical angle.

Ultraviolet (UV) Radiation refers to electromagnetic radiationcharacterized by a vacuum wavelength shorter than that of the visibleregion, but longer than that of soft X-rays. Ultraviolet radiation maybe subdivided into the following wavelength ranges: near UV, from about380 nm to about 200 nm; far or vacuum UV (FUV or VUV), from about 200 nmto about 10 nm; and extreme UV (EUV or XUV), from about 1 nm to about 31nm.

Vacuum Wavelength refers to the wavelength electromagnetic radiation ofa given frequency would have if the radiation were propagating through avacuum and is given by the speed of light in vacuum divided by thefrequency.

Visible Light refers to electromagnetic radiation characterized by avacuum wavelength shorter than that of the IR radiation, but longer thanthat of UV radiation, the visible range is generally regarded to be fromabout 400 nm to about 700 nm.

INTRODUCTION

Many prior art refractive index sensors that are based on measurementsof critical angle encapsulate the light source and the photodetectorarray in the material, e.g., transparent epoxy, that forms the prism.One drawback to prior refractive index sensors is that they typicallyuse only one light source that provides one wavelength of light used forrefractive index measurements. This has several disadvantages. First,the single light source can limit the range of angles of incidence andtherefore the range of refractive index that can be measured. Secondly,a single light source can limit the resolution of sensor.

Another drawback to prior art refractive index sensors is that the lightsource and photodetector array are combined with the prism in anintegrated design in which an optical epoxy encapsulates both the lightsource and the photodetector array. Designs that use epoxy encapsulationsuffer from degradation of epoxy starting at temperatures of about 85C.°, limiting the range of processes that can be measured. Certainexisting sensor designs use refractive index-matching plastic prismsthat are chemically incompatible with many fluids or chemicals. Suchdesigns require an intervening, chemically compatible material at themeasurement interface. In addition, plastic prisms preclude the use ofshort wavelength light.

Another disadvantage arises from the fact that often the interface withthe sample is not a face of the prism but is instead a “window” made ofglass or other optically dense material that is glued to one of thefaces of the prism. However, there is a significant mismatch incoefficient of thermal expansion (CTE) between optical epoxies used forthe prism in prior refractive index sensors and the typical material(e.g., borosilicate glass) that is used for the window. For example, atypical optical epoxy has a CTE of about 50 parts per million percentigrade degree (50 ppm/C.°). Borosilicate glass has a CTE of about 7ppm/C.°, which is about seven times smaller. A common optical grade ofborosilicate glass is sold commercially under the name Schott BK-7.

The CTE mismatch between the window and the optical epoxy can lead toproblems when the fluid being sampled is at a temperature that issignificantly hotter or colder than room temperature.

According to embodiments of the present invention an optical sensorapparatus may include features that overcome the disadvantages of priorart refractive index sensors.

Optical Sensor Apparatus

According an embodiment of the present invention an optical sensorapparatus of a new design uses an optical waveguide structure made of aprecision-machined, unitary, optically transparent material, whichsimultaneously forms a measurement interface and interfaces with lightsources and a photodetector array.

The use of a light guiding structure made from a precision-machinedoptically transparent material allows the use of a wider range ofwavelengths than is possible with a plastic prism. The solidprecision-machined optically transparent material allows for the directdeposition of a reflecting material onto the light guiding structure,eliminating the need to mechanically place a mirror on a prism. Thisreduces design complexity and improves optical signal.

The two common-wavelength light sources (e.g., yellow LEDs)significantly extend the RI range that can be measured compared toprevious RI sensor designs. Light from the two common-wavelength lightsources overlaps in a center of the RI range, which increases signal tonoise ratio.

FIG. 1A and FIG. 1B illustrate examples an optical sensor apparatus 100according to an embodiment of the present invention. The optical sensorapparatus 100 is based on a reflection geometry. The apparatus 100generally includes a light guiding structure 102 made of an opticallydense material such as borosilicate glass, or sapphire. Alternatively,the light guiding structure 102 may be made of quartz, diamond, undopedyttrium aluminum garnet (YAG), calcium carbonate, or any other opticallytransparent material. The light guiding structure has at least threefaces F1, F2 and F3. The light guiding structure 102 may be attached toa printed circuit board 104 by any suitable means, e.g., mechanicalattachment, epoxy, fusing, etc. Two or more light sources 106A, 106B,106C, 106D and a photodetector array 108 are attached to the printedcircuit board 104. By way of example and not by way of limitation, eachlight source may be a light emitting diode (LED). Other non-limitingexamples of light sources include solid state lasers and semiconductorlasers. By way of example, and not by way of limitation, the lightguiding structure 102 may be in the form of a prism made of an opticallytransmissive medium or material, as shown in FIG. 1A and FIG. 1B.However, embodiments of the invention are not limited to those thatutilize a prism to provide the desired light guiding function. Othergeometries and components may be used to provide the desired reflectiongeometry for the sensor apparatus 100.

The photodetector array 108 is generally a position sensitive detectorthat any array of light sensing elements can produce signals that varydepending on how much light is received at different parts of the array.The signals may be analog or digital electrical signals. By way ofexample and not by way of limitation, the photodetector array may be aphotodiode array. Alternatively, an array of charge coupled devices,photoresistors, and arrays of other types of light sensing elements maybe used in the photodetector array. In general, each light sensingelement in the array may provide a signal that corresponds to theirradiance (optical power per unit area) at that element. Each lightsensing element thus provides a irradiance signal for a corresponding“pixel”, e.g., as shown in FIG. 2. An optional memory 110, e.g., in theform of an integrated circuit, may be coupled to the photodetector array108 to temporarily store the pixel signals produced by the photodetectorarray. By way of example, and not by way of limitation, the memory 110may be a flash memory or electrically erasable programmable read-onlymemory (EEPROM).

As seen in FIG. 1B, light from the light sources is emitted over a rangeof angles. The light emitted from the light sources passes through thefirst face F1. At least some of the light that passes through the firstface F1 undergoes total internal reflection at an interface with asample 111 proximate the second face F2. The first face F1 may be coatedwith an anti-reflection (AR) coating. The light that is totallyinternally reflected at the interface is sometimes referred to herein as“totally internally reflected light”. In the example depicted in FIG.1A, a window 112 is attached to the second face F2 and the interface isthe side of the window that is in contact with the sample. It is notedthat the window 112 is optional. If the window is omitted, as in theexample depicted in FIG. 1B, the interface 113 with the sample 111 maybe located at the second face F2.

The totally internally reflected light from the interface with thesample is reflected at a third face F3 and passes back through the firstface F1 to the photodetector array 108. The prism 102 thus maps some ofthe cone of light from each of the light sources 106A, 106B, 106C, 106Donto the photodetector array 108. The index of refraction of the sampleand the offset between the light sources determines which portion ofeach cone of light will be totally internally reflected at the interfacewith the sample.

In some embodiments, the material of the prism 102 may be selected suchthat total internal reflection takes place at the third face F3.Alternatively, the third face F3 may be coated with a metal ordielectric reflective coating to facilitate reflection of the lightincident on the third face F3 from inside the prism 102.

To facilitate computation of the index of refraction, the apparatus 100may further include a processor 114 coupled to the photodetector array108 and/or the memory 110. The processor 114 may also be coupled to thelight sources 106A, 106B, 106C, 106D and may be configured toselectively control which light source is turned on and which lightsource is turned off. The processor 114 may be configured, e.g., byprogramming with suitable executable instructions 115, to analyze theirradiance pattern measured by the photodetector array and determine thecritical angle at the interface with the sample and the correspondingindex of refraction. Specifically, the processor may analyze theirradiance pattern to determine the pixel location of a telltale featurein the pattern indicative of light reflected at the interface with thesample 111 at the critical angle. The pixel location of the telltalefeature may then be correlated to the index of refraction either byanalysis from first principles from the known geometry and materialproperties of the components of the apparatus 100 or from a simplecalibration using measurements of one or more materials of knownrefractive index.

The critical angle may be determined from the irradiance pattern asfollows. For angles of incidence below the critical angle, some lightwill be refracted at the interface with the sample 111 into the sampleand some will be reflected to the photodetector array 108. At thecritical angle, the refracted light is refracted along the interface.For angles greater than the critical angle all of the light is reflectedat the interface with the sample 111. The light rays corresponding tolight reflected at the critical angle can be identified by a transitionbetween low intensity and high intensity in the pattern of irradiance atthe photodetector array. The pixel location of the transition can becorrelated to critical angle from the known geometry and refractiveindices of the light guiding structure 102 and window 112 and from theknown locations of the light source(s) and the photodetector array 108.Alternatively, the pixel location of the transition may be calibratedagainst refractive index using several samples of known refractiveindex. The light sources 106A, 106B, 106C, 106D may include two or morelight sources that emit light at a common wavelength (referred to hereinas “common-wavelength light sources”) and/or two or more light sourcesthat emit light at different wavelengths. By way of example, and not byway of limitation, the optical sensor apparatus 100 may include fourlight emitting diodes. Two LEDs may be configured to emit light of acommon wavelength and two other LEDs may be configured to emit light ofdifferent wavelengths.

Using two light sources that emit light at the same wavelength allowschanging scale so that the photodetector array 108 can be overfilled,i.e., filled with totally internally reflected light beyond the extentpossible with a single light source. The common-wavelength light sourcesmay be configured such that cones of totally internally reflected lightfrom the different common-wavelength light sources overlap at thephotodetector array to some extent. The use of two common-wavelengthlight sources provides for a larger refractive index range. The extracommon-wavelength light source gives a greater number of resolvablerefractive indices that can be detected, which can provide greaterrefractive index range or better resolution or a combination of bothdepending on how the common-wavelength light sources are configured tofill the photodetector array.

For example as shown in FIG. 1B, suppose that light sources 106B and106C are the common-wavelength light sources. The cone of light fromlight source 106B that undergoes total internal reflection at theinterface is indicated by dashed lines. The cone of light from lightsource 106C that undergoes total internal reflection at the interface isindicated by dotted lines. In this example, the cones of totallyinternally reflected light from the two light sources 106B, 106C aremapped to two corresponding regions of the photodetector array labeledRB and RC. Because of the different locations of the light sources 106A,106B, light from these two sources is totally internally reflected atthe interface with the sample 111 over different ranges of incidentangles. These different ranges of incident angles translate intodifferent patterns of irradiance at the photodetector array. If thepositions of the light sources 106B, 106C, the geometry and refractiveindex of the prism 102, are known, it is possible to determine the indexof refraction for the sample 111 by analyzing the irradiance pattern atthe photodetector array 108, as discussed above.

In the apparatus 100 depicted in FIG. 1B, light sources 106A and 106Dmay emit light at vacuum wavelengths different from each other and alsodifferent from the vacuum wavelength of the light emitted bycommon-wavelength light sources 106B, 106C. In one particularnon-limiting implementation the two common wavelength LEDs may both emityellow light, corresponding to a vacuum wavelength of about 589 nm. Onenon-common wavelength LEDs may emit ultraviolet light, e.g., at a vacuumwavelength of about 375 nm and the other non-common wavelength LED mayemit infrared light, e.g., at a vacuum wavelength of about 940 nm.

By including two or more light sources that emit light at two or moredifferent wavelengths the apparatus 100 can be used to estimate opticaldispersion of the sample. Since the dispersion of a material is aproperty characteristic of the type of material, measuring dispersioncan be used distinguish between one material and another. By way ofexample, the processor 114 may be configured, e.g., by suitableprogramming, to determine an optical dispersion of the sample 111 byanalyzing irradiance measurements obtained by the photodetector array108 when light from the light sources 106A, 106D that is totallyinternally reflected at the interface with the sample 111. The lightsources 106A, 106D may be turned on at the same time or mayalternatively be turned on one at a time for sequential measurements. Itis noted that by using multiple light sources emitting at differentwavelengths the apparatus 100 may avoid the need for optical filters toobtain different wavelengths from a single light source. Eliminating theneed for optical filters reduces design and mechanical complexity, andimproves power efficiency. A more compact design is also possible byeliminating optical filters. In addition signal to noise ratio (SNR) canbe improved by using multiple light sources. A larger range ofwavelengths is possible with multiple light sources because the(wavelength) range of a single light source may be limited.

There are a number of different ways in which optical dispersionmeasurements made with the sensor apparatus 100 may be used. By way ofexample, and not by way of limitation, if the sensor apparatus 100 isused to measure the refractive index of a solution having a knownsolvent (e.g., water (H₂O)) and known solute (e.g., hydrogen peroxide(H₂O₂)) measurements of refractive index n versus vacuum wavelength maybe used to estimate the concentration of solute.

In certain embodiments of the present invention there may be afree-space gap g between light sources 106A, 106B, 106C, 106D and theprism 102 or between prism 102 and the photodetector array 108 or both.The free-space gap allows some flexibility in the design of the sensorapparatus since the photodetector array and light source are not bothencapsulated by the prism material. The free space gap also allows anadditional degree of flexibility in optimizing the irradiance fillpattern at the photodetector array for multiple light sources. Inaddition, a free-space gap (e.g., an air gap) is less susceptible todegradation than epoxy.

The light guiding structure 102 may be made of a rigid optical material,such as sapphire, BK7, or an undoped garnet such as undopedYttrium-Aluminum Garnet (YAG). If used without a window 112, therefractive index of the light guiding structure 102 must generally belarger than the highest refractive index that the apparatus 100 isexpected to measure. Alternatively, if a window is used, it may bedesirable for the refractive index of the window 112 to be greater thanthe refractive index of the light guiding structure 102 in order toavoid total internal reflection at the interface between the prism andthe window. However, this is not always the case. For example, a freeelectron metal may be placed at the interface 111 making the sensorapparatus 100 a surface plasmon resonance sensor, which as a practicalmatter, may act as a refractive index sensor. Furthermore, the adhesiveused to attached the window 112 to the light guiding structure 102 maybe one that has a higher refractive index than the material of thestructure 102 but a lower refractive index than the material of thewindow 112. In some embodiments, it may also be desirable for theadhesive to have a refractive index that is higher than the largestrefractive index that the apparatus 100 is expected to measure.

The window 112 is optional, but for many applications it is preferred.The light guiding structure 102 can be glued directly to the window 112or vice versa, e.g., using a suitable optical adhesive. Alternatively,the window may be attached to the light guiding structure (or viceversa) using a mechanical seal with a refractive index matched gel oroil. The light guiding structure may alternatively be fused to thewindow. The material of the light guiding structure 102 may be chosen tobe CTE-matched to the material of the window 112. By way of example, andnot by way of limitation, the prism may be made of borosilicate glasshaving a CTE of 7.1 ppm/C.° and sapphire having a CTE in the c-plane(the plane perpendicular to the c-axis) of 4.5 ppm/C.°. In such a case,the CTE of the prism would be about 1.6 times larger than the CTE of thewindow, which is sufficiently small that the prism and window can besaid to be CTE matched.

It is further noted for the adhesive used to attach the window to theprism to be sufficiently complaint to accommodate the CTE of differencebetween the prism and window materials. By way of example, and not byway of limitation, for a prism made of borosilicate glass and a sapphirewindow, a suitable UV-curing polymer adhesive is sold commercially underthe name Norland Optical Adhesive 61 (or NOA 61), and is available fromNorland Products of Cranbury, N.J. It is further noted that NOA 61 has arefractive index between that of borosilicate glass and sapphire.

It is also desirable for the prism material to be CTE-matched to thematerial of the printed circuit board 104. By way of example, theprinted circuit board may be made of a glass reinforced epoxy compositematerial such as FR4, which as a CTE of about 11 ppm/C.°, which issufficiently close to the CTE of borosilicate glass to be consideredCTE-matched for the purposes of embodiments of the present invention.

A number of variations are possible on the embodiments described above.Two possible variations, among others, are shown in FIG. 3A and FIG. 3B.As shown in FIG. 3A, in an optical sensor apparatus 300 light sources306A, 306B and a photodetector array 308 may be located in anapproximately co-planar configuration proximate a first face F1 of alarge light guiding structure 302. The first face F1 may be coated withan anti-reflection (AR) coating. Light from the sources 306A, 306Bpasses through the first face F1 toward a window 312 attached to asecond face F2. Total internal reflection occurs at an interface 313between the window 312 and a sample 311 over corresponding ranges ofangles of incidence for each light source 306A, 306B. A portion of thetotally internally reflected light is reflected at a third face F3 andpasses back through the first face F1 to the photodetector 308. If theprism 312 is made of a material with a relatively high refractive index,e.g., about 1.7 or greater, light totally internally reflected at theinterface 313 may also be totally internally reflected at the third faceF3. Alternatively, a metallic or dielectric reflective coating may beformed on the third face F3.

The prism 302 may be made from a number of different materials of highrefractive index. By way of example, and not by way of limitation, theprism 302 may be made may be cut from a sapphire wafer in a roughlytriangular shape and the edges of the triangle may be polished toprovide the faces F1, F2, and F3. To avoid two overlapping responsesfrom each light source due to birefringence it may be desirable toorient the sapphire such that its optical axis (the so-called c-axis) isoriented perpendicular to the plane of the wafer from which the prism isformed.

The larger size of the prism 302 accommodates a larger lateral spacingD1 between the light sources 306A, 306B and a large lateral spacing D2between light source 306B and the photodetector array 308. The largersize of the prism 302 and the large spacing D1 allows for a relativelysmall amount of overlap in the ranges of incidence angles for whichlight from the light sources 306A, 306B is totally internally reflectedat the interface 313 but still allows each range of incidence angles tofill the photodetector array 308. Spreading the incidence angle rangesfor each light source over the entire photodetector array allows forbetter resolution of refractive index since the incidence angles fortotal internal reflection at the interface 313 and the correspondingrefractive indices are spread out over a greater number of pixels. Theparticular geometry shown in FIG. 3A allows for such improved resolutionwhile allowing the light sources 306A, 306B and the photodetector arrayto be at roughly the same height relative to the plane of the commonsupport (not shown). By way of example, the common support may be aprinted circuit board, like the PCB 104 of FIG. 1A and FIG. 1B. For someapplications it may be desirable to use a smaller light guidingstructure while keeping a relatively large lateral spacing D1 betweenthe light sources 306A, 306B and a large lateral spacing D2 between thelight source 306B and the photodetector array. If the size of thephotodetector array 308 remained the same as in FIG. 3A and thephotodetector array and the light sources 306A, 306B were at the sameheight, this would lead to poorer resolution in refractive index sinceeach range of incidence angles for total internal reflection at theinterface 313 would be spread over a smaller number of pixels. However,this problem may be overcome by offsetting the relative height of thephotodetector array 308 by a gap g relative to a height of light sources306A, 306B. This puts the photodetector array 308 further from the firstface F1 of the prism 302 as shown in the apparatus 300′ depicted in FIG.3B. In such a case, the desired range of refractive index may bemeasured without sacrificing resolution and without requiring that thelight sources and photodetector array be at the same relative height.

It is noted that the optical sensor apparatus 300 depicted in FIG. 3Aand the optical sensor apparatus 300′ depicted in FIG. 3B may includeother components described above, such as additional light sources, amemory, processor, and software. These components have been omitted fromthe drawing for the sake of clarity. Furthermore, although two lightsources 306A, 306B are depicted in FIG. 3A and FIG. 3B, those skilled inthe art will recognize that more than two light sources may be used.Furthermore, the light sources may include two or more common-wavelengthlight sources or two or more light sources that emit light at differentvacuum wavelengths or some combination of these configurations of lightsources.

Optical sensors of the type described herein have numerous advantagesover competing concentration sensing technologies such as opticalabsorption. For example, because light is reflected rather thantransmitted there are no problems with measuring the refractive indexfor opaque fluid samples. Furthermore, the sensor apparatus can be usedwith any material for the window. For example, in certain applications,such as pharmaceutical manufacturing, disposable bags made oftransparent plastic may be used to provide the window.

In addition, calibration of a sensor apparatus of the type describedherein is much easier than for an absorption spectroscopy sensor. If thesample under test is relatively simple it is not necessary to measurespeciation. Calibration of the species of interest can be done byperforming an auto-titration of the species of interest while measuringindex of refraction with the sensor as a function of speciesconcentration as determined by the auto-titration. By taking aderivative of irradiance versus pixel position for the calibrationsample it is possible to determine refractive index of the calibrationsample to within a small fraction of a pixel position. A run index ofrefraction measurements versus pixel position can be done for a seriesof samples of known concentration and the resulting calibration can bestored in memory 110. An offset in the calibration may be shifted byperforming a run of measurements of irradiance versus pixel position fora reference sample, such as de-ionized water and re-zeroing thecalibration.

A sensor apparatus of the type described herein can therefore competewith absorption spectrometry in the near IR or UV-visible wavelengthranges. The reflection geometry in the embodiments described herein alsoprovides substantial advantages over sensors that use transmissiongeometry. For example, diffraction and adsorption effects can beeliminated, and the refractive indexs of opaque fluids can be measured.

While the above is a complete description of the preferred embodimentsof the present invention, it is possible to use various alternatives,modifications, and equivalents. Therefore, the scope of the presentinvention should be determined not with reference to the abovedescription but should, instead, be determined with reference to theappended claims, along with their full scope of equivalents. Anyfeature, whether preferred or not, may be combined with any otherfeature, whether preferred or not. In the claims that follow, theindefinite article “A” or “An” refers to a quantity of one or more ofthe item following the article, except where expressly stated otherwise.The appended claims are not to be interpreted as includingmeans-plus-function limitations, unless such a limitation is explicitlyrecited in a given claim using the phrase “means for”. Any element in aclaim that does not explicitly state “means for” performing a specifiedfunction, is not to be interpreted as a “means” or “step” clause asspecified in 35 USC §112, ¶6.

What is claimed is:
 1. An optical sensor apparatus, comprising: anoptically transmissive light guiding structure having planar first,second, and third faces; two or more light sources located outside thelight guiding structure adjacent the first face; a photodetector arraylocated outside the light guiding structure adjacent the first face,wherein the light guiding structure, light sources, and photodetectorarray are configured such that light from the two or more light sourcesthat is totally internally reflected at an optical interface between thelight guiding structure and a sample outside the light guiding structureproximate the second face is reflected at the third face and incident ona portion of the photodetector array that depends on a refractive indexof the sample, wherein the two or more light sources are positioned withrespect to the light guiding structure and photodetector array such thatlight from each of the two or more light sources that is totallyinternally reflected at the interface and reflected at the third facecorresponds to a different range of refractive index of the sample andmaps to a corresponding portion of the photodetector array.
 2. Theapparatus of claim 1, wherein the two or more light sources areseparated from the light guiding structure by a free-space gap.
 3. Theapparatus of claim 1, wherein the photodetector array is separated fromthe light guiding structure by a free-space gap.
 4. The apparatus ofclaim 1, wherein the two or more light sources include two or morecommon wavelength light sources configured to emit light of a commonvacuum wavelength, wherein the two or more common wavelength lightsources are positioned with respect to the light guiding structure andphotodetector array such that light from each of the two or more commonwavelength light sources that is totally internally reflected at theinterface and reflected at the third face corresponds to a differentrange of refractive index of the sample and maps to a correspondingportion of the photodetector array.
 5. The apparatus of claim 1, whereinthe two or more light sources include two or more light sourcesconfigured to emit light of different corresponding vacuum wavelengths.6. The apparatus of claim 5, further comprising a processor coupled tothe photodetector array, wherein the processor is configured todetermine an optical dispersion of the sample from measurements obtainedby the photodetector array with light from the two or more light sourcesconfigured to emit light of different corresponding vacuum wavelengthsthat is totally internally reflected at the interface.
 7. The apparatusof claim 1, wherein the two or more light sources, light guidingstructure and photodetector array are configured such that light fromthe one or more light sources that is totally internally reflected atthe interface passes through the first face a first time between leavingthe one or more light sources and total internal reflection at theinterface and passes through the first face a second time between thetotal internal reflection at the interface and arriving at thephotodetector array.
 8. The apparatus of claim 1, further comprising anoptical window attached to the second face of the light guidingstructure, wherein the optical window is characterized by a refractiveindex that is greater than a refractive index of the light guidingstructure.
 9. The apparatus of claim 8, wherein the light guidingstructure is made of a material that is CTE-matched to the material ofthe window.
 10. The apparatus of claim 8 wherein the light guidingstructure is made of borosilicate glass and the window is made ofsapphire.
 11. The apparatus of claim 1, wherein the light guidingstructure is configured such that light from the two or more lightsources that is totally internally reflected at the optical interface istotally internally reflected at the third face.
 12. The apparatus ofclaim 10, wherein the two or more light sources and the photodetectorarray are at substantially the same height relative to a supportingstructure.
 13. The apparatus of claim 10, wherein the two or more lightsources are at substantially the same height relative to a supportingstructure and a height of the photodetector array is offset with respectto the height of the two or more light sources.
 14. The apparatus ofclaim 10, wherein the light guiding structure is made from a sapphirewafer.
 15. The apparatus of claim 13, wherein an optical axis of thesapphire window is oriented perpendicular to a plane of the sapphirewindow.